1. Field of the Invention
The present invention relates to a mesh structure and a field-emission electron source apparatus using the same.
2. Description of Related Art
In recent years, with the development of fine processing technology for semiconductors, attention has been directed to a vacuum microelectronics technology of integrating a large number of minute cold cathode structures on the order of micrometers on a semiconductor substrate or the like. Field-emission electron source arrays including the minute cold cathode structures obtained by such a technology achieve flat-type electron emission characteristics and a high electric current density, and do not require a heat source such as a heater, unlike hot cathodes, thus offering potential as electron sources for a low-power-consumption next-generation flat display, sensors and electron sources for a flat-type imaging apparatus.
As vacuum apparatuses using the field-emission electron source arrays described above, field-emission electron source display apparatuses shown in JP 9(1997)-270229 A, JP 9(1997)-69347 A, JP 6(1994)-111735 A and JP 2000-251808 A, field-emission electron source imaging apparatuses shown in JP 2000-48743 A, etc. and a light-emitting device shown in JP 2002-313263 A have been known.
In general, as shown in FIG. 26, such a field-emission electron source apparatus using a field-emission electron source array includes a front panel 101, a back panel 105 and a wall part 104, which are fixed firmly by a sealing material 109 such as frit glass or indium. An inner space of the field-emission electron source apparatus is maintained under vacuum.
An inner surface of the front panel 101 is provided with an anode electrode 102 transmitting incident light from outside, for example, and a surface of the anode electrode 102 is provided with a target 103. In general, the target 103 is a phosphor layer in which phosphors emitting three colors of light are arranged regularly when used as a field-emission electron source display apparatus and a photoelectric conversion film for converting incident light into a signal charge when used as a field-emission electron source imaging apparatus.
An inner surface of the back panel 105 is provided with a semiconductor substrate 106 on which a field-emission electron source array is formed. A plurality of cold cathode elements (emitters) 107 and peripheral elements 108 including an insulating layer formed so as to surround the individual cold cathode elements 107 and gate electrodes for applying a voltage for drawing electrons from the cold cathode elements 107 are integrated in the field-emission electron source array. Electron beams emitted from the cold cathode elements 107 are made to land on the target 103, whereby the phosphor can be caused to emit light so as to display an image in the field-emission electron source display apparatus and an image formed on the photoelectric conversion film by incident light can be read in the field-emission electron source imaging apparatus.
A representative example of the field-emission electron source generally can be a Spindt-type field-emission electron source in which cold cathode elements with a sharpened tip are formed on a semiconductor substrate, an insulating layer is formed around the cold cathode elements, gate electrodes are formed on the insulating layer, and a voltage is applied between the cold cathode elements and the gate electrodes, thereby emitting electrons from the tips of the cold cathode elements. Besides the above, examples thereof include field-emission electron sources of an MIM (metal insulator metal) type in which an insulating layer is formed between cathode electrodes and gate electrodes, and a voltage is applied to the insulating layer, thereby emitting electrons by a tunnel effect; those of an SCE (surface conduction electron source) type in which a minute gap is provided between cathode electrodes and emitter electrodes, and a voltage is applied between these electrodes, thereby emitting electrons from the minute gap; and those using a carbonaceous material such as DLC (diamond like carbon) or CNT (carbon nanotube) for an electron source.
In these field-emission electron sources including a cold cathode, the amount of electrons emitted from individual cold cathode elements is minute. Therefore, in the case where they are used as a field-emission electron source display apparatus or as a field-emission electron source imaging apparatus, unit cells each including a plurality of the field-emission electron sources (electron source cells) are formed, thus securing an amount of electric current necessary for performing a predetermined operation.
These cells are arranged on a flat surface, for example, in a matrix. More specifically, a plurality of emitter lines extending along a longitudinal direction are arranged at regular intervals in a transverse direction, a plurality of gate lines extending along the transverse direction are arranged at regular intervals in the longitudinal direction, and the cell is provided at each intersection of these plurality of emitter lines and gate lines. When driving the field-emission electron source apparatus, the emitter lines and the gate lines are selected sequentially, whereby an electron beam is emitted sequentially from the cell at the intersection of the emitter line and the gate line that are selected. In the instant specification, the cell that emits an electron beam as described above will be referred to as a “selected cell” in the following. In this manner, an image can be displayed in the field-emission electron source display apparatus, and a formed image can be read in the field-emission electron source imaging apparatus.
Since the field-emission electron source performs the field emission of electrons by a strong electric field formed between the cold cathode elements and the gate electrodes, the electrons are emitted from the individual cold cathode elements while having a predetermined divergence (the angle of this divergence is called a “divergence angle” and, for example, is about 30° in the case of the Spindt-type field-emission electron source).
Generally, in the vacuum apparatuses using the field-emission electron source described above, as shown in FIG. 26, the field-emission electron source array is placed on the back panel 105 of a vacuum container, and the target 103 on which an electron beam from the field-emission electron source array is landed for performing a predetermined operation is formed on the front panel 101. Here, the distance from the field-emission electron source array to the target 103 is determined uniquely by the distance between the back panel 105 and the front panel 101 on which they are provided.
In other words, in these conventional field-emission electron source apparatuses, the distance between the field-emission electron source array placed on the back panel 105 and the target 103 formed on the front panel 101 varies considerably from an ideal design distance depending on the accuracy of a portion where the front panel 101 and the back panel 105 are joined to the wall part 104.
For example, when the front panel 101 and the back panel 105 are joined to the wall part 104 using frit glass, variations of an amount of frit glass to be supplied, shrinkage generated in the course of burning and welding at about 400° C., etc. cause variations of the distance between the field-emission electron source array placed on the back panel 105 and the target 103 formed on the front panel 101.
Also, when the front panel 101 and the back panel 105 are joined to the wall part 104 by low-temperature sealing using a soft metal such as indium, since the indium is squashed between the front panel 101 and the wall part 104 and between the wall part 104 and the back panel 105 at the time of sealing, the variations of the supply amount and the squashing amount of the indium cause variations of the distance between the field-emission electron source array placed on the back panel 105 and the target 103 formed on the front panel 101.
The variations of the distance between the field-emission electron source array and the target 103 can be in a range of about several hundred micrometers to several millimeters.
As described above, in the conventional field-emission electron source apparatuses, it is difficult to control the distance between the field-emission electron source array placed on the back panel 105 and the target 103 formed on the front panel 101 in a highly accurate manner. Further, the electrons are emitted from each of the cold cathode elements with the divergence angle of about 30°. Therefore, the variations of the distance between the field-emission electron source array and the target 103 lead to variations of a degree of expansion of an electron beam spot (namely, a spot diameter) formed on the target 103 by the electron beam. This is very disadvantageous for the field-emission electron source display apparatuses and the field-emission electron source imaging apparatuses in which there is a demand for a uniform image.
Also, in order to achieve a high-definition field-emission electron source display apparatus and a high-definition field-emission electron source imaging apparatus, the size of the cells on the field-emission electron source array has to be reduced sufficiently. In this case, the distance between the field-emission electron source array and the target 103 also has to be reduced sufficiently, and further, its error has to be controlled within about several tens of micrometers, for example. However, in the conventional field-emission electron source apparatuses, since the distance between the field-emission electron source array and the target 103 may have variations of about several hundred micrometers to several millimeters, it is difficult to achieve the high-definition field-emission electron source display apparatus and the high-definition field-emission electron source imaging apparatus.
Moreover, in the conventional field-emission electron source apparatuses, the front panel 101 is subjected to an outside pressure and warped when the vacuum container is evacuated. Since the target 103 is formed on the inner surface of the front panel 101, when the front panel 101 is warped, the distance from the field-emission electron source array differs between a central portion and a peripheral portion of the target 103. As a result, the diameter of the electron beam spot formed on the target 103 differs between the central portion and the peripheral portion of the target 103.
Consequently, a difference in quality of an image to be displayed arises between a center of a screen and a peripheral portion of the screen in the case where the field-emission electron source apparatus is used as the field-emission electron source display apparatus, and a difference in quality of an image to be captured arises between a center of a screen and a peripheral portion of the screen in the case where the field-emission electron source apparatus is used as the field-emission electron source imaging apparatus.
Unlike the apparatus shown in FIG. 26, vacuum apparatuses using a field-emission electron source array in which a shield grid electrode is provided between the field-emission electron source array and a target are illustrated in JP 9(1997)-270229 A and JP 2000-48743 A.
FIG. 27 is a sectional view showing a field-emission electron source apparatus used as a field-emission electron source imaging apparatus illustrated in JP 2000-48743 A.
A vacuum container 118 includes a light-transmitting front panel 115, a back panel 117 and a wall part 116 also serving as a spacer portion for holding a meshed shield grid electrode 120. The front panel 115, the back panel 117 and the wall part 116 are fixed firmly by a sealing material 133 made of frit glass and a sealing material 119 made of indium. The inside of the vacuum container 118 is maintained under vacuum.
An inner surface of the front panel 115 is provided with a photoelectric conversion target 114 including an anode electrode 113 transmitting incident light 111 from outside and a photoelectric conversion film 112 formed on the surface of the anode electrode 113.
An inner surface of the back panel 117 is provided with a field-emission electron source array 129 including cold cathode elements 124, a cathode conductor 125 for supplying an electric potential to the cold cathode elements 124, an insulating layer 126 formed on the cathode conductor 125 so as to surround the cold cathode elements 124 and gate electrodes 128 disposed on the insulating layer 126 so as to surround the cold cathode elements 124.
The shield grid electrode 120 is disposed between the photoelectric conversion target 114 and the field-emission electron source array 129. The shield grid electrode 120 is supplied with a voltage higher than that applied to the gate electrodes 128.
The shield grid electrode 120 includes a plurality of through holes 120a. The plurality of through holes 120a and the plurality of cold cathode elements 124 are in a one-to-one correspondence with each other, and the centers of the through holes 120a are located immediately above the respective tips of the cold cathode elements 124 for emitting electron beams.
From the tip of the cold cathode element 124, the electron beam is emitted with the divergence angle of about 30°. In this electron beam, only a partial electron beam that is emitted in a substantially upright direction passes through the through hole 120a of the shield grid electrode 120 corresponding to this cold cathode element 124 and reaches the photoelectric conversion target 114, and the rest of the electron beam that is emitted obliquely is absorbed by the shield grid electrode 120.
JP 2000-48743 A mentions that the divergence of the electron beam reaching the photoelectric conversion target 114 can be reduced in this manner.
However, in order to allow only the partial electron beam that is emitted in the substantially upright direction in the electron beam emitted from the cold cathode element 124 to reach the photoelectric conversion target 114, the relative relationship among the size of the cold cathode elements 124, the distance between the adjacent cold cathode elements 124, the distance from the field-emission electron source array 129 to the shield grid electrode 120, the opening diameter of the through holes 120a of the shield grid electrode 120 and the thickness of the shield grid electrode 120 have to be designed strictly.
For example, when the size of the cold cathode elements 124 and the distance between the adjacent cold cathode elements 124 are reduced, it becomes necessary to reduce the distance between the adjacent through holes 120a of the shield grid electrode 120 as well. However, in this case, there is a possibility that the electron beam emitted from the tip of the cold cathode element 124 with the divergence angle of about 30° passes through not only the corresponding through hole 120a disposed immediately above this cold cathode element 124 but also other through holes 120a near this through hole 120a and reaches the photoelectric conversion target 114.
Further, in the case of increasing the distance from the field-emission electron source array 129 to the shield grid electrode 120, there also is a possibility that the electron beam emitted from the tip of the cold cathode element 124 with the divergence angle of about 30° passes through not only the corresponding through hole 120a disposed immediately above this cold cathode element 124 but also other through holes 120a near this through hole 120a and reaches the photoelectric conversion target 114.
Also, when the number of the cold cathode elements 124 is larger than the number of the through holes 120a of the shield grid electrode 120, the center of the through hole 120a is not located immediately above the tips of part of the plurality of cold cathode elements 124 formed in the field-emission electron source array 129. Thus, there is a possibility that the partial electron beam that is emitted in the substantially upright direction in the electron beam emitted from this cold cathode element 124 cannot pass through the through hole 120a and is absorbed by the shield grid electrode 120, and the partial electron beam that is emitted obliquely passes through the through holes 120a at positions other than immediately above this cold cathode element 124 and reaches the photoelectric conversion target 114.
As described above, unless the relative relationship of the dimensions of individual constituent members is designed strictly, it is not possible to reduce the divergence of the electron beam reaching the photoelectric conversion target 114. Consequently, there arises a problem that the size of an imaging pixel increases.
Further, in the case where an attempt is made to apply the field-emission electron source apparatus illustrated in FIG. 27 to a flat-type imaging apparatus for capturing an image of VGA (640 dots×480 dots, horizontally by vertically), the following problems may arise.
In the flat-type imaging apparatus for VGA, 640 dots of pixels and 480 dots of pixels are arranged horizontally and vertically, and the total number of pixels is about 310,000 dots. Assuming that 100 cold cathode elements 124 are arranged in one pixel (dot), the total number of the cold cathode elements 124 is about 31,000,000, which is huge. In the case of a 1-inch (2.54-cm)-diagonal (outer size) flat imaging apparatus, the horizontal size of the field-emission electron source array is 1.275 cm, and the vertical size of the field-emission electron source array is 0.956 cm, so that the size of a single dot is 0.02 mm (=20 μm). For arranging 100 cold cathode elements 124 like lattice points in this single dot, 10 cold cathode elements 124 have to be arranged in one direction. In order to form the through holes 120a in the shield grid electrode 120 so as to achieve a one-to-one correspondence with the cold cathode elements 124, the through holes 120a have to have an inner diameter of not greater than 2 μm.
In this case, it is considered possible to form the through holes 120a having an inner diameter of not greater than 2 μm by setting the shield grid electrode 120 to have a thickness of not greater than 1 μm. However, if the thickness of the shield grid electrode 120 is not greater than 1 μm, the shield grid electrode 120 is very likely to have problems of insufficient strength and warping. On the other hand, if the thickness is set to be greater than 1 μm, it is considered impossible to form the through holes 120a having an inner diameter of not greater than 2 μm in a sheet of metal such as nickel, copper or aluminum, which is described as the material for the shield grid electrode 120 in JP 2000-48743 A. Overall, it is considered nearly impossible to form the through holes 120a that are in a one-to-one correspondence with the cold cathode elements 124 in the shield grid electrode 120.
Even if the through holes 120a having an inner diameter of not greater than 2 μm could be formed and the problems of insufficient strength and warping could be solved, there would be a further problem that the field-emission electron source array 129 and the shield grid electrode 120 need to be aligned in a highly accurate manner.
In other words, in order to arrange the centers of the through holes 120a immediately above the tips of the cold cathode elements 124 without any displacement, the relative positional relationship between the field-emission electron source array 129 and the shield grid electrode 120 has to be controlled at an accuracy within 0.1 μm. However, the general assembling accuracy at present has a limit of about 1 μm. In view of this, it also is considered difficult to achieve a flat-type imaging apparatus using the field-emission electron source apparatus illustrated in FIG. 27.
Also, in the case where the single cold cathode element 124 is made to correspond to a single pixel as shown in FIG. 27, it is necessary to supply an amount of electric current necessary to operate the single pixel from the single cold cathode element 124. However, in view of the fact that current emission characteristics of the field-emission cold cathode element are on the order of nanoampares, it is considered difficult to achieve a field-emission electron source apparatus in which only a single cold cathode element 124 is arranged in a single pixel.
Conversely, when a plurality of cold cathode elements 124 are made to correspond to a single pixel and a single through hole 120a of the shield grid electrode 120 is made to correspond to the single pixel, the tips of the cold cathode elements 124 corresponding to this through hole 120a are located at positions other than that immediately below the center of the through hole 120a. Therefore, as described above, there is a possibility that the partial electron beam that is emitted in the substantially upright direction in the electron beam emitted from this cold cathode element 124 cannot pass through the through hole 120a and is absorbed by the shield grid electrode 120, and the partial electron beam that is emitted obliquely passes through the through holes 120a constituting the adjacent pixels and reaches the photoelectric conversion target 114. Accordingly, in this case, it also is considered difficult to achieve a field-emission electron source apparatus. In other words, with the field-emission electron source apparatus shown in FIG. 27, it is very difficult to allow the electron beam from the cold cathode element 124 to pass through only the through hole 120a arranged immediately above this cold cathode element 124 and reach the photoelectric conversion target 114 in an efficient manner.
Furthermore, the field-emission electron source apparatus illustrated in FIG. 27 has another problem described below.
As illustrated in FIG. 27, the insulating back panel 117 provided with the field-emission electron source array 129 and the front panel 115 provided with the photoelectric conversion target 114 opposed to this field-emission electron source array 129 are joined to each other with the wall part 116 interposed between their outer peripheral portions, such that the inside of the vacuum container 118 is maintained under high vacuum.
At this time, by providing frit glass having a low melting point serving as the sealing material 133 between the back panel 117 and the wall part 116 and burning it at about 400° C., the back panel 117 and the wall part 116 are attached to each other, so that the inside of the vacuum container is maintained airtight. Also, when the shield grid electrode 120 is positioned and fixed onto a step portion 121 of the wall part 116, frit glass having a low melting point is used. Therefore, the distance between the field-emission electron source array 129 and the shield grid electrode 120 depends on the thickness of the low-melting frit glass between the back panel 117 and the wall part 116 and that of the low-melting frit glass between the step portion 121 of the wall part 116 and the shield grid electrode 120.
Accordingly, variations are generated in the degree of parallelity and the distance between the field-emission electron source array 129 and the shield grid electrode 120.
As a result, the degree of divergence of the electron beam on the photoelectric conversion target 114 (focusing characteristics) varies for every field-emission electron source apparatus, or the degree of divergence of the electron beam varies depending on the position on the photoelectric conversion target 114 even within a single field-emission electron source apparatus. Thus, in the case where the field-emission electron source apparatus is used as a field-emission electron source imaging apparatus, a captured image varies for every apparatus, and partial variations occur in a captured image.
Moreover, the field-emission electron source apparatus illustrated in FIG. 27 has another problem described below.
The shield grid electrode 120 disposed between the field-emission electron source array 129 and the photoelectric conversion target 114 is like a thin film and produced, for example, by fixing a thin film copper mesh to a metallic holding frame under a tension or by forming a film of a metal such as Ni, Cr, Cu, Ag or Co or an alloy thereof on a surface of an insulating material such as glass or ceramics provided with a large number of through holes by vapor deposition, sputtering, chemical plating or the like.
However, the shield grid electrode 120 produced by fixing a thin film copper mesh to a metallic holding frame under a tension is likely cause variations in a tension distribution. For example, when the tension differs between a central portion and a peripheral portion of the copper mesh, the shape of the through holes 120a and the distance between the adjacent through holes 120a differ between the central portion and the peripheral portion of the copper mesh. In such cases, it becomes difficult to arrange the centers of the through holes 120a immediately above the respective tips of all the cold cathode elements 124 in the field-emission electron source array 129. In other words, the relative positions of the tips of the cold cathode elements 124 and the centers of the through holes 120a are displaced partly, thus causing the amount of the electron beam reaching the photoelectric conversion target 114 to differ between the central portion and the peripheral portion of the photoelectric conversion target 114 or to vary locally. Therefore, variations occur in a brightness distribution in the case where the field-emission electron source apparatus is used as a field-emission electron source display apparatus, and a captured image becomes nonuniform in the case where it is used as a field-emission electron source imaging apparatus.
Further, in the shield grid electrode 120 produced by forming a film of a metal or an alloy on a glass surface provided with a large number of through holes, the glass itself has adsorbed much gas. Even if the above-noted metal film is formed on the glass surface, gas is emitted easily due to the impact of an electron beam while driving the field-emission electron source apparatus.
Also, when a thinner glass sheet is prepared and provided with a large number of minute through holes 120a, there arises a problem that the mechanical strength of the glass deteriorates remarkably, so that the glass sheet becomes easy to crack. In particular, for allowing a large amount of the electron beam to reach the photoelectric conversion target 114, the distance between the adjacent through holes 120a has to be reduced, resulting in a still lower mechanical strength.
Moreover, in the shield grid electrode 120 produced by forming a film of a metal or an alloy on a ceramic surface provided with a large number of through holes, there are problems that the ceramic needs a burning process, the distance between the adjacent through holes 120a is difficult to reduce, the mechanical strength is insufficient similarly to the case of using glass, etc.
Although the shield grid electrode provided in the field-emission electron source apparatus has been discussed in the above description, mesh structures having a plurality of through holes represented by such a shield grid electrode also are used for various applications other than the above-described field-emission electron source apparatus. For example, by making the thickness of the mesh structure sufficiently larger than the diameter of the through holes, the mesh structure can be used as a collimator that allows atoms, molecules, light or the like to pass from one surface to the other surface, thus providing their traveling direction with directivity; or by adjusting the diameter of the through holes in the mesh structure, the mesh structure can be used as a particle filter for screening particles by particle diameter.
Similarly to the above, the mesh structures used in such applications often are produced by forming through holes in a base material such as metal, glass or ceramics. However, the use of such a base material leads to the following problems.
That is, in the case of using metal as the material for the mesh structure, holes have to be drilled deeply in the metal structure. However, it is difficult to use a current hole processing technique using a die or the like, and a small diameter of the through hole cannot be achieved by that technique.
Also, in the case of using glass as the material for the mesh structure, hole processing is difficult, and there are concern about gas emission in a vacuum and the problem of insufficient mechanical strength, similarly to the above.
Further, in the case of using ceramics as the material for the mesh structure, the following problem occurs. That is, in order to secure the mechanical strength, it is difficult to reduce the intervals between the through holes. Also, since a large number of the through holes cannot be formed, the amount of atoms, molecules or light on an exit side becomes considerably lower than that on an incident side when the mesh structure is used as a collimator. Additionally, ceramics generally develop dimensional variations due to burning and thus make it difficult to control the through hole dimensions in a highly accurate manner. Therefore, the ceramics are not suitable for filters whose through hole diameter has to be determined in a highly accurate manner with respect to the size of particles.